“Not only that, but the difference between the Big Six standard tariffs and the cheapest deals is getting smaller and that usually means one thing: an impending hike in prices to the millions of customers on these standard variable deals.”

He added that households should switch now to a fixed tariff, before prices rise even further.

The warning comes as the number of people switching suppliers hit a three-year high.

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According to Energy UK, which represents the industry, almost 600,000 customers moved supplied in October and 9 per cent of people went to a small supplier.

Lawrence Slade, chief executive of Energy UK, said: “It only takes a few minutes to find the right deal. More than 40 suppliers are offering many deals under £1,000, so take advantage.”